1. Field of the Invention
The present invention relates to an apparatus for measuring the power of a nuclear reactor, such as a light-water reactor (LWR), and to a method for manufacturing such an apparatus.
2. Discussion of the Background
In a conventional light-water reactor, such as a boiling-water reactor, the power of the reactor is measured by detecting the neutron flux within the reactor. Neutron sensors which have been used for this purpose are roughly divided into two systems comprising a local power range monitoring system (hereinafter referred to as "LPRM") serving as a fixed fission ionization chamber in the reactor, and a traversing incore probe system (hereinafter referred to as "TIP") serving as a movable fission ionization chamber.
The LPRM, which is a system at a fixed position in the reactor, employs a uranium isotope as a substance for reacting with neutrons in the reactor. As a result, the LPRM has the following disadvantages: the uranium isotope is consumed as the use of the LPRM proceeds, thereby involving the risk that sensitivity to neutrons may be decreased, and accordingly, the measurement precision of the system may be deteriorated. Since the neutron flux varies among various locations in the reactor at which the LPRM sensors are provided, the use of the system involves great variations in sensitivity among various locations in the reactor.
In view of these disadvantages, the TIP is also used during operation of the reactor, such as a boiling-water reactor, in order to calibrate the sensitivity of the LPRM with respect to the neutron flux, so that variations in the sensitivity of the LPRM sensors can be calibrated.
In order to use the TIP for the purpose of calibrating the sensitivity of the LPRM sensors, however, a large-scale drive mechanism for moving the TIP within the reactor is required to be accommodated at a location below the reactor pressure vessel. The drive mechanism requires great labor for its maintenance. In addition, when the TIP is taken out of the reactor, handling of the TIP, which is activated, involves the risk of the operators may be exposed to radiation.
For these reasons, it has recently been conceived to omit the use of the TIP, and use, instead of the TIP, another type of reactor power measurement apparatus in combination with the LPRM. This type of apparatus, which is referred to as a .gamma.-thermometer, comprises a system of sensors at a fixed position in the reactor that does not require a drive mechanism, nor does it involve substantial deterioration of sensitivity.
The .gamma.-thermometer is a type of reactor power measurement apparatus which detects the quantity of heat attributable to radiation (.gamma.-rays). Specifically, the .gamma.-thermometer has a structure of stainless steel or a like material which is disposed in the sensor sections of the .gamma.-thermometer and which is capable of generating heat by absorbing energy as a result of the absorption or inelastic scattering of radiation (specially .gamma. rays) within the reactor, and also has thermocouples or the like for measuring the temperature distribution formed when the generated heat is transferred to an external coolant. In contrast with a fission ionization chamber, the .gamma.-thermometer does not, in principle, involve sensitivity deterioration.
FIG. 9 to FIG. 11 show the construction of a sensor section of a .gamma.-thermometer serving as a conventional power measurement apparatus for a nuclear reactor, the distribution of temperature in the axial direction of a core tube of the apparatus, and the arrangement of thermocouples and a heater in the sensor section. The reactor power measurement apparatus forms an elongated, rod-shaped structure having a plurality of such sensor sections in the axial direction.
The sensor section has a double-tube construction comprising a core tube 1 and a cover tube 2. A gap 3 is provided between the core tube 1 and cover tube 2 around the circumference of the interface of these tubes 1 and 2, the gap 3 serving as an adiabatic portion for providing temperature distribution with a great variation range. Heat is transferred from the core tube 1 to the cover tube 2 through surfaces of contact therebetween. The distribution of temperature in the axial direction of the core tube 1 is such that when the sensor section generates no heat, temperature distribution is flat, as indicated by broken curve a in FIG. 9, whereas when the sensor section generates heat due to radiation in the reactor, temperature T rises in the center of the axial dimension of the gap 3, as indicated by solid curve b.
This is because, in the reactor power measurement apparatus, when the core tube 1, irradiated by .gamma. radiation, generates heat, the heat is conducted from the core tube 1 to the cover tube 2 through a pair of routes that bypass upper and lower portions of the gap 3 which are respectively above and below the center of the gap 3 in the axial direction. The increment in temperature .DELTA.T at the axial center of the gap 3 with respect to temperature at other locations of the gap 3, corresponds to calorific power, that is, a radiation dose or fuel power in the vicinity of the sensor section. Thus, measurement of such temperature increment .DELTA.T enables the measurement of the power of the relevant nuclear reactor.
In order to measure such temperature increment .DELTA.T, a plurality of thermocouples 5 of a number corresponding to the number of the sensor sections, as well as a heater 6 for calibrating the sensitivity of the sensors, are received in an inner bore 4 formed in the inner tube 1. As shown in FIG. 11, each of the thermocouples 5 is coated with an insulating coating 7a and a metal coating 8a, while the heater 6 is coated with an insulating coating 7b and a metal coating 8b.
In order that the conventional power measurement apparatus for a nuclear reactor can accurately measure the temperature of the core tube 1, the metal coatings 8a of the thermocouples 5 must tightly contact the core tube 1. In the manufacturing of a power measurement apparatus for a reactor having the above sensor construction, however, since the metal coatings 8a of the thermocouples 5 cannot be tightly contacted with the core tube 1 by brazing or soldering the metal coatings 8a to the core tube 1, the following process is adopted: the thermocouples 5 and the heater 6, each coated with the insulating coating 7a or 7b and the metal coating 8a or 8b, are inserted into the inner bore 4 of the inner tube; thereafter, the core tube 1 is swaged or caulked to bring the core tube 1 into press contact with the metal coatings 8a of the thermocouples 5; and finally, the cover tube 2 is fitted on the outer surface of the core tube 1, and swaged to press-contact the cover tube 2 on that outer surface.
This process, however, has some problems. When the swaging of the core tube 1 is so insufficient that the metal coatings 8a of the thermocouples 5 are not tightly contacted with the inner surface of the core tube 1, and the temperature cannot be accurately measured. When the core tube 1 is so strongly caulked that the strong force applied to the core tube 1 causes deformation of the insulating coatings 7a and the metal coatings 8a of the thermocouples 5, this may cause problems in the apparatus, such as disconnection or insulation failure.
Similar problems may be caused in the conventional apparatus due to the arrangement in which, in order to calibrate the sensitivity of each sensor, the heater 6 received in the inner bore 4 is caused to heat the core tube 1, so that a temperature distribution approximating the temperature distribution formed by heat generation due to radiation, is formed in the core tube 1. In the conventional arrangement, since the thermocouples 5 are disposed in the vicinity of the heater 6, the thermocouples 5 may be heated excessively, and this involves the risk of disconnection, insulator failure, etc.